A QUASI-BIENNIAL OSCILLATION SIGNAL IN GENERAL-CIRCULATION MODEL SIMULATIONS

The quasi-biennial oscillation (QBO) is a free atmospheric mode that a ffects the equatorial lower stratosphere. With a quasi-regular frequen cy, the mean equatorial zonal wind alternates from easterly to westerl y regimes. This oscillation is zonally symmetric about the equator, ha s its largest amplitude in the latitudinal band from 20-degrees-S to 2 0-degrees-N, and has a mean period of about 27 months. The QBO appears to originate in the momentum deposition produced by the damping in th e stratosphere of equatorial waves excited by diabatic thermal process es in the troposphere. The results of three 10-year simulations obtain ed with three general circulation models are reported, all of which sh ow the development in the stratosphere of a QBO signal with a period a nd a spatial propagating structure that are in good agreement with obs ervations without any ad hoc parameterization of equatorial wave forci ng. Although the amplitude of the oscillation in the simulations is st ill less than the observed value, the result is promising for the deve lopment of global climate models.